Operation of x-ray tubes



OPERATION OF X-RAY TUBES Filed July 19, 1951 /N V[ N TOP V/bfor QuifinerBY 4- saw A TTOPNEY i States Patenrfi p age across a load, by derivingits gridbias from a poten- 'tiometer connected across the load, the tubebeing put next the positive terminal'of the load I V "connected-tothe'hig'h'tension supply-,and. therefore,-usu- V v furtherdisadvantageis that as the'regulation of the X-ray inals by the aid of two switchtubes oneoneach sidevof 2,730,528 Pat'nteiLJan. 10, .1956.

1 2 voltage controllerashallbeat low potential. ZTothis ends-:- v the.poteritiometenmay be connected not across the X-ray 2,730,628 tube, butbetweenone of its -electrodes ;and.earth... lt is bettertto provideindependentautomatic potential-stabilisationon eachaelectrode .byusingapairof otentiometersconnecting the X-r ay tube anode and cathoderespectively. to earth. .Ea ch potentiometer is tapped near its .earthedcnd,-and thevoltages between these tappings and earth'are.-balanced..against two equalstabilised standard D. C. voltages, thevalues of which are: adjusted simultaneously by handto select. the .tubevoltage..- Asthe two standard voltages.arekeptexactlyequal .this systemnot only stabilise's' thetotaltube voltage but alsoensuressymmetric-distribution of thisvoltage with reference to earth evenifthetwo switch tubes or auxiliary switch tubes are nOLaliktL-r Analternative method-is to use-a-fixed comparison voltage andshift thetapping of the potentiometer for voltage selection. But'as it isdesirable to have the'potentiometervn as a. whole. immersed in .oil (tomake the .voltageratio independent of temperature variation) thefirst-mentioned methodis preferable.

'Ihe'present..invention is not-.limitedto anyparticular meansv oftransmitting. the control from the low potential. controlleito the highpotential switch tubes.

A preferred schemetor this purpose is described in the specificationofmy co -pending application 237,560,filed"-- July" 19, 1951',now..Patent- No. 2,659,016, and is shownin a simplified form in 'theaccompanying diagram of con nections.

In ZthisLdiag ram .the.X-ray tube is indicated by '1, ands. V p H pthe'two switcht'ubes by 2 and3 respectively. For conl -pfiiisrelecvtrfldes cannot be Y trolling X-ray tubes which have only totakea small cur tli r mi mvtm P- to earth Y, rent, .for instance for"tubes used .for; crystallographicin 'Tlie pms t n e t m s a s e of p a gn- -raw vesti'gat'io'n's, the" switch tubes may be triodes. As arule'j.tube, h is to say switching it on and Off and cqmrollme 35 it willflbeconvenientto use beam tetrodes as indicated in" the value of andkeepifig'i'constariti the'i'voltage at its termlh jdi grqni'o pentodesfor" the 1 sake of their.highe mplificatio'n. Between 'the X-ray tubeelectrodes and. earth oPEiwrioN oF X-RxY TUnEs Victor? Quittnelgwlmndon, England assignor "to General Radiological Limited, London,England, aBntish companies;

A plie'alidnquly"19,1951;Serial Nd.237,559- ciata's. noat ,a snsaonGreat Marathi/222, 1950 4 claims. 02 250 403 Joa'rnar'er'the"Fraiiklin"Institute;" vol. 214, pages "155-10 162, and-since widelyused, to maintainconstant the volt- If an X-ray, tube be operated by theaid. of such aswiteh a tubeQthe switching off of the x-ra tube leavesits cathode t ally -at alhigh neg ative voltage with respect to earthi-eA tube- No lt a'geitakes'- place on one side of the X-ray tubethe',the voltage applied to the X-ray tube. vided by a potentiometerconnected across the X-ray tube. It is desirable that the manuallyoperated part of the the X-ray tube." Thiss'cheine has the advantagesthat the X-ray'tube is wholly disconnected from the high tension circuitwhen not in use, theelectrode potentials are symmetrical with respect toearth whatever the difference between them, and the switch tubes need bebuilt to withstand only half the supply voltage.

The switch tubes must be capable of carrying the maximum currentrequired in the X-ray tube while it is working.

The grid voltage of each switch tube relative to its cathode must bevariable between a negative value sufiiciently high to render the tubenon-conductive under at least one half of the high-tension supplyvoltage and a less negative or positive value at which the full currentof the X-ray tive gridvoltages are indispensable, and, in this case, an

auxiliary switch tube with a separate voltage source is usually provided'in order to supply the grid current required by themain control tube.This auxiliary switch 2 tube acts at thesame time as an amplifier and soincreases the sensitivity of the control system.

vTo switch the X-ray tube on and off and to select the tube voltage amanually operated control device is provided which brings about therequisite changes in the grid voltage of the switch tubes. If the X-raytube voltageis required to remain constant, notwithstanding variationsof mains voltage, and independent of the value of the tube current, thecontrol device must include, in addition to the manual means ofadjustment, an element dependent on This can be proare"connectedpotentiometers or tapped resistances 4, 5, and 6, 7. Thesewill usually be immersed in oil so that is is inconvenient to vary thetappings. Instead the voltage drawn from each potentiometer is balancedagainst a steady but adjustable low voltage drawn from a source 8, 9respectively; these sources have a common earthed terminal. The means ofadjustment of the two sources are ganged so that a single control knobon the control panel of the apparatus serves to vary both tocorresponding extents.

The control voltages so obtained in a low potential circuit, in part bymanual setting of the sources 8, 9, and in part from the X-ray tubeitself, are required to govern the bias of the switch tube in a highpotential circuit. The particular means of transmitting the control hereillustrated is the subject of my application 237,560. The controlvoltages are employed to govern the output of high requency generators10 and 11, in any manner usual in modulation for communication purposes.In the scheme shown the voltage across the resistance 4 is applied tothe grid and the voltage of the source 8 to the cathode of a tube, forinstance an amplifying tube 101, in the high frequency generator 10, sothat the diflference of the two voltages is the grid bias of the tube.Tube 101 amplifies the output of an oscillator comprising a tube 102having a tunable anode circuit 103 coupled to its grid in wellknownmanner. Similarly the difference between the voltage across resistance 6and that of source 9 could govern the output of the correspondinglynumbered generator 11. The generators 10, 11 supply transformers 12, 13respectively, which are high frequency transformers with adequateinsulation between their windings to withstand the potentials at whichthe switch tubes operate. The transformers apply grid bias to therespective switch tubes through'rectifiers 14, 15 and smoothing elements17, 18, 19 and 20, 21 and 22.

But the voltages applied to the generator 10 are of opposite sign tothose applied to the generator 11; yet a chance increase in the voltagebetween either X-ray tube electrode and earth must increase the negativebias on the corresponding switch tube. This may be taken care of eitherby including a source of steady negative bias in the grid circuit'of oneswitch tube, or by making the generators 10, 11 difier in the respectthat increase in the positive tapped bias of the one and increase in thenegative tapped bias of the other alike increase the generator output.In the diagram shown a reversing tube 114 changes the sign of the biasapplied to tube 111.

Whatever the means employed to transfer the control from a low potentialcircuit to the X-ray tube circuit, it is important that it shouldintroduce as little delay as possible or an oscillation of X-ray tubevoltage may be set up.

I claim:

1. A circuit for operating an X-ray tube comprising a hightension sourceof direct current, a grid-controlled switch tube, an X-ray tube and asecond grid-controlled switch tube connected in series in that orderacross the high tension source, the cathode of one tube being joined tothe anode of its neighbour, like resistances connected between therespective electrodes of said X-ray tube and earth, tappings on saidresistances near their earthed ends, and bias means controlled by thepotential of said tappings and connected to the grid and cathode of therespective switch tubes to apply between them a voltage varying with thepotential of the adjacent X-ray tube electrode.

2. A circuit for operating an X-ray tube comprising a high tensionsource of direct current, a grid-controlled switch tube, an X-ray tubeand a second grid-controlled switch tube connected in series in thatorder across the high tension source, the cathode of one tube beingjoined to the anode of its neighbour, like resistances connected betweenthe respective electrodes of said X-ray tube and earth, tappings on saidresistances near their earthed ends,

high frequency generators having their output controlled 40 byconnection to said tappings, and high frequency transformers havingtheir primary windings connected to the respective high frequencygenerators, and their secondary windings to the cathode-grid circuits ofthe respective switch tubes, said circuits including each a rectifierand smoothing elements.

3. A circuit for operating an X-ray tube comprising a high tensionsource of current, an X-ray tube and two grid-controlled control tubesone on each side of the X-ray tube, all connected in series, apotentiometer connected across said X-ray tube, manually variable meansfor producing a bias voltage, and means for applying to the grids ofboth said control tubes a bias dependent on the difference between thevoltage tapped from said potentiometer and the voltage produced by saidmanually variable means.

4. A circuit for operating an X-ray tube comprising a high tensionsource of current, an X-ray tube and two grid-controlled control tubesone on each side of the X-ray tube, all connected in series, apotentiometer connected across said X-ray tube and earthed at its midpoint, tappings on opposite sides of said earthed mid point, means forapplying to one control tube a bias voltage varying with the potentialof one of said tappings, and means for applying to the other of saidcontrol tubes a bias voltage reversed in sign and varying with thepotential of the other of said tappings.

References Cited in the file of this patent UNITED STATES PATENTS2,036,069 Morrison Mar. 31, 1936 2,132,175 Machlett Oct. 4, 19382,217,939 Bischoff Oct. 15, 1940 2,227,353 Kuntke Dec. 31, 19402,240,478 Bischoff May 6, 1941 2,475,197 Quittner July 5, 1949 2,488,168Brown Nov. 15, 1949 2,545,247 Vingerhoets Mar. 13, 1951 2,605,430 MarcyIII July 29, 1952 FOREIGN PATENTS 778,520 France Sept. 15, 1934

